This invention relates to adaptive optics control systems for use with high power lasers and more particularly to a two-wavelength control system capable of generating synthetic long wavelength error signals for minimizing 2.pi.N ambiguity.
Lasers having a chemical gain medium such as hydrogen fluoride or deuterium fluoride are attractive for many potential high power applications because of their short wavelength and high power capabilities. However, the gain medium of the chemical laser inherently has a low density and thus requires a large volume and it operates efficiently only on many simultaneous transitions which are chemically related but optically independent. Large scale annular resonators suitable for use with chemical lasers and capable of extracting good quality beams at high efficiency are difficult to fabricate. The utilization of a master oscillator with a plurality of amplifiers driven in parallel with the oscillator is one method of obtaining high power output. However, the efficient utilization of the output beam from such an oscillator-amplifier configuration requires that the phase distribution of the output of each amplifier be matched. Additionally, potential problems are encountered because of multi-line operation in which lasing can switch, at high frequency, between different optical transitions in the gain profile the resulting phase fluctuations and distortions of the multiple output beams are difficult to compensate for. Additionally, anomalous dispersion causes the local index of refraction within each of the optical amplifiers to vary away from the line center across the gain profile. Thus, differential beam steering occurs between the different frequency beams passing through the optical amplifiers resulting in a reduction in the optical quality of the output beam. These degradation effects can produce significant reduction in the far field beam intensity if the gain profiles are dominated by inhomogeneously broadened gain profiles.
Remote control of optical path length within an amplifier-oscillator configuration or within an oscillator configuration forms the basis of a field of adaptive optics which includes the control of deformable mirrors, the alignment and phasing of multi-mirror systems, and the control of laser phase arrays. An acquisition problem common to all of these adaptive optics systems is the 2.pi.N ambiguity or equalization of path length . In either case, the acquisition problem results from an initial optical path difference across the system aperture which is appreciably greater than the optical wavelength of the laser radiation. A single wavelength adaptive controller, as is known in the prior art, will match path lengths to within an integral number of wavelengths resulting in 2.pi.N ambiguity. This results in an optical system that converges to a state of non-zero optical path difference producing a degradation of the system performance in most cases of interest. To assure that the phase of each laser transition is matched from all amplifiers in the far field, it is necessary to equalize the optical path length from the oscillator exit through the amplifiers to a transmitter to within a small fraction of a wavelength. This would be extremely difficult to accomplish with a non-adaptive mirror system. However, in an adaptive system utilizing a system of independently controlled mirrors, optical path equalization is possible.
Warren in a Contract Report, SAMSO-TR-78-50, dated Feb. 16, 1978, discloses an adaptive optics system in which two lasing lines of a multi-line oscillator are sensed using a dispersive element to separate two lasing frequencies from the output of the oscillator. The beams having the two different frequencies are given different modulation frequencies and are incident onto a beat frequency detector. The use of different frequency modulation of different laser lines allows the comparison of the phase of one line passing through one amplifier with the phase of the same line from a second amplifier and the adjustment of a control mirror to bring the beams into phase. Additionally, the phase difference between the output from two amplifiers on a second line can be compared and the control mirror adjusted to bring the beams on the second laser line into phase. These steps are repeated for the radiation passing through all of the amplifiers within an optical system to obtain a combined output beam having all components in phase. Preferably the radiation passing through one amplifier is utilized as a standard for the comparison of all other radiation for phase matching the wavefront distribution emanating therefrom. As suggested by Warren, the important criteria is to make the optical path lengths through the amplifiers the same, and only two lines need to be sensed for phase matching. Once the equal length criteria is established, any line hopping in the oscillator will not destroy the phase-matched property of each line passing through the optical amplifiers. However the acquisition and control of the information to minimize or eliminate the optical path difference between the radiation passing out of each optical amplifier is difficult because of the 2.pi.N ambiguity problem.